The present invention relates generally to exposure, and more particularly to exposure apparatuses and methods, device fabricating methods, and devices fabricated from an object to be exposed or a target object. The exposure apparatus and method are used to fabricate various devices including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pick-up devices such as CCDs, as well as minute contact hole patterns used for micromechanics. Here, the micromechanics is technology for applying the semiconductor IC fabricating technique for fabrications of a fine structure, thereby creating an enhanced mechanical system that may operate at a level of micron.
A photolithography process uses an exposure apparatus to transfer a mask pattern onto a photosensitive material (resist) which is applied to a silicon wafer, glass plate, etc. (simply called “wafer” hereinafter), and includes steps of an application of resist, exposure, development, etching and a removal of the resist. For the exposure in this series of steps, resolution, overlay accuracy and throughput are three important factors. The resolution is the minimum size for a precise transfer. The overlay accuracy is precision in overlaying multiple patterns on a wafer. The throughput is the number of sheets processed per unit of time.
The fabrication of a device using the lithography technique has employed a projection exposure apparatus that uses a projection optical system to project a pattern drawn on a mask or reticle (these terms are used interchangeably in this application) onto a wafer, thereby transferring the pattern. The projection optical system enables diffracted beams from the pattern to interfere on a wafer and forms an image. The normal exposure enables 0-th order and list order diffracted beams (namely, three beams) to interfere with each other.
Mask patterns include an adjacent and cyclic line and space (L & S) pattern, an adjacent and cyclic contact hole pattern, and isolated contact holes that are non-adjacent and isolated, but to transfer a pattern with high resolution, it is necessary to select optimal exposure conditions (such as illumination conditions, exposure light amounts, etc.) in accordance with kinds of patterns.
The resolution R of a projection exposure apparatus is given by using a wavelength λ of a light source and the number of apertures NA in a projection optical system in the following Rayleigh equation:                     R        =                              k            1                    ×                      λ            NA                                              (        1        )            where k1 is a constant determined by a development process and others. In a normal exposure case, k1 is approximately 0.5–0.7.
The recent demand for highly integrated devices have increasingly required minute patterns to be transferred or high resolution. Although the above equation shows that the higher numerical aperture NA and decreased wavelength λ would be effective for the higher resolution, improvements of these factors have already reached the limit at the current stage. Thus, it is difficult for normal exposure to form a pattern of less than 0.15 μm onto a wafer. Accordingly, it has been suggested to employ the phase shift mask technology that enables two beams out of those diffracted beams which have passed through the pattern to interfere with each other, thus forming an image. The phase shift mask reverses by 180° phases of adjacent light-transmitting portions on it, and cancels out the 0-th order diffracted beam, thus enabling two ±1st order diffracted beams to interfere with each other and forming an image. Use of this technique would decrease k1 in the above equation down to substantially 0.25, thus improving the resolution R and forming a pattern of less than 0.15 μm onto a wafer.
However, although the conventional phase shift mask technique may be effective for such a simple pattern as a cyclic L&S pattern, it has had a difficulty in exposing an isolated pattern and an arbitrarily complicated pattern with high exposure performances (i.e., with high resolution, overlay accuracy, and throughput). In particular, the recent semiconductor industry has been shifting its production to a system chip which includes highly value-added and various types of patterns, and thus it has become necessary to form more than one kind of contact hole pattern on a mask.
On the other hand, as is disclosed in Japanese Laid-Open Patent Application No. 11-143085, it is conceivable to use double exposures (or multiple exposures) with two masks to expose different kinds of patterns separately, but the conventional double exposures require two masks and incur many practical disadvantages: That is, this approach results in an increased cost and lowered throughput because of two exposure steps, as well as requiring high overlay accuracy for two mask exchanges.
Therefore, it is an exemplary object of the present invention to provide an exposure method and apparatus that can expose mask patterns with high resolution and without exchanging the mask, the mask patterns, with a minute line width (e.g., less than 0.15 μm), which include a mixture of various patterns ranging from an L&S pattern to an isolated and a complicated pattern.